CODEWITHSUNDEEP
c++arrayspointersvector
jimmu
jimmu

Reputation: 1056

Converting vectors to an array

I am having trouble converting type vector< vector > to an array.So far, I've tried to do the following (with help from /u/ Robert Crovella)

pos_x_h=(double *)malloc(N*sizeof(double));
pos_y_h=(double *)malloc(N*sizeof(double));
for (int i = 0; i<N; i++){
  vector<double> temp = r[i];
  pos_x_h[i] = temp[i][0];
  pos_y_h[i] = temp[i][1];
}

Here, r is the position vector with N elements each having x and y components. I also tried doing

double arr[N];
std::copy(r.begin(), r.end(), arr);

Both attempts didn't work, and I'm not sure why. You can see the code here.

Upvotes: 0

Views: 70

Answers (1)

Floris
Floris

Reputation: 46435

The following ought to work. Note that I prefer sizeof *pos_x_h over sizeof(double) since the former makes sure the size is correct even if you change the type of the variable (which might be in another piece of code). updated* for C++ you need to cast the result of malloc. I was thinking with my C hat on...

second update

A bit more thought tells me you really don't want to have temp as a vector - that is just making things more confusing. Instead, point to the address of the first element of r[i]: this compiles without errors

    pos_x_h=(double *)malloc(N*sizeof(double));
    pos_y_h=(double *)malloc(N*sizeof(double));
    for (int i = 0; i<N; i++){
      double* temp;
      temp = &r[i][0];
      pos_x_h[i] = temp[0];
      pos_y_h[i] = temp[1];
    }

Of course you could simply do

    pos_x_h=(double *)malloc(N*sizeof(double));
    pos_y_h=(double *)malloc(N*sizeof(double));
    for (int i = 0; i<N; i++){
      pos_x_h[i] = r[i][0];
      pos_y_h[i] = r[i][1];
    }

and avoid the whole mess.

Upvotes: 2

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